Internal conversion between bound states and the Pauli exclusion principle

We present the results of an investigation of the contribution to decay of the nucleus by an unexplored mode of internal conversion in which an excited nuclear state deexcites with promotion of an electron to an intermediate filled bound state. This process, which apparently violates the Pauli exclusion principle, may make a significant contribution to the nuclear decay rate when very close matching of the atomic and nuclear transitions is achieved. For an M1 transition, we show that Pauli-forbidden bound internal conversion (PFBIC) transition may occur by excitation of an inner electron to an initially occupied outer orbital. Numerical calculations of the PFBIC process are presented for the decay of the 77.351 keV level of Au-197, in the neutral atom and in ions up to 69(+). The PFBIC process is found to have a maximum probability in Au-197 ions with charge state of approximately 40(+), due to close energy matching of the nuclear transition to the 1s --> 3s electronic excitation. The expected dependence of the nuclear lifetime on PFBIC is analyzed.